Phase-modulation system



I 3 "Shets-Sheet 1 B. D. LoUGHLlN PHASE-MODULATIUN SYSTEM Filed March s, 1942.

vMay 28,

BY ATro NEY- a. n. LouGHLlN PHASE-MODULATION SYSTEM l Filed March 3, 1942 IIYII lllll May 28, 1946.

mvENTo' ,.WD www BY AURNEY Il JV... .lll JIQII Ill41l la lnuunod i B. D. LoUGHLlN 2,401,007

PHASE-MODULATION SYSTEM Filed March 3, 1942 3 Sheets-Sheet 3 May 2S, 1946.

mvENTon BERNARD D.LOUGH LIN BY W A'IT RNEY fe( pressed by the action o! the modulator.

Patented May 28, 1946 une Sars s rivisti 'r ori-ice rss-MonoLATroN sr'sTEM Bernard D. Loughlin, East Drange, N. J., assigner,

by 'mesne assignments, to Hazeltlne Research, Inc., Chicago, Ill., a corporation o! Illinois Application March 3, 1942, Serial No. 433,219 i9 Claims. (Cl. 179-1715) The presentinvextion relates to .phase-modulation systems and. particularly, to such systems suitable for use' in rier-wave transmitters. K In frequency-modulation carrier-wave transmitters, a signal to be transmitted is applied to a modulator stage oi' the transmitter so to modulate a carrier wave thereof that the frequency of range of frequency deviation, .by the method 'of Y frequency multiplication, the latter wave being thereupon suitable for radiation by the transl maier.

.. In a particular' form of the prior art transmitter arrangement described, the phasemodulator stage comprises two carrier-wave translating channels having common` input and output circuits. A relatively low-frequency carrier wave is applied to the input circuit of the modulator stage and is directly translated through a 90-degree phase-shifting network to theV output circuit of fthe stage. The other translating channel includes a balanced Imoduphase-modulation carfrequency-modulated carrier wave suitable for radiation by the transmitter. This arrangement has the disadvantage that only approximate linearity of phase modulation can be obtained and this only when the maximum degree of phase modulation is of the order of plus or minus one-half radian or less. This type of phase-modulation system also requires that the carrier wave applied to the; modulator stage have a relatively low frequency. There is, consequently, thev further disadvantage with this system that it requires an unnecessarily large number oi frequency-multiplier stages by which thelow-Irequency modulated carrier wave may be changed to the high-frequency carrier wave required for radiation byI the transmitter. This greatly and undesirably adds to the complexity and initial costei the transmitter and to the operating and maintenance costs thereof.

It is an obiectof the present invention, therefore, to provide a new and improved 'phasea modulation system which avoldsone o r more of the `disadvantages and limitations of the prior art' systems of this nature. l

It is an additional object of the invention to provide a phase-modulation system in which the linearity of phase modulation is limited only by the degree of linearity which can be attained in the wave form oi' saw-tooth oscillations used in the system for phase-modulation control and,v -thus, one characterized by a high degree of 'linearity of phase modulation.-

' It is a further object of the invention to provide a phase-modulation system particularly lator having one inputcircuitto which thel car-- rler wave is applied and anotherc input circuit `to which the modified o'r integrated modulation signal is applied. The balanced modulator is so arranged that there are developed in its output circuit only amplitude-modulati'on lsidebanda the carrier-wave component being sup- The sldeband components are added to the 90-dearee phase-shifted carrierwave in the. output circuit of the modulation stage to develop in this output circuit a carrier wave which may be considered as being phase-modulated in accordance with a distorted'modulation signal or frefluency-modulated in accordance with the original'modulation signal. This modulated carrier VVwave isthen applied to suitable frequency-multiplier stages of the transmitter which increase the range oi.' frequency deviation of the carrier.

. wave' to provide a relatively high-frequency suitable for use in a frequency-modulation transmitter in-whlch the degree of frequency multiplication oi' the modulated carrier wave necessary to obtain a high-frequency frequencymodulated carrier wave is greatly reduced, thereby to eil'ect a material slmpliilcation of the transmitter arrangement with consequent` reduction of initial cost and operating and maintenance costs. Y

In accordance with the invention, a .phasemodulation system comprises an input circuit adapted to have applied thereto modulation signals, means forv generating reference oscillations, and means li'or generating comparative oscillations of the mean frequency-oi the reierv encev oscillations.- The system also includes :means responsive to the relative phase of corresponding cycles of the reference and comparative oscillations for deriving a, control eect, -n-ieans responsive to the control .ellect tending to maintain a predetermined constant phase relaltion between the reference and comparative oscillations, and means responsive to the modulation signals for modifying the action of the control effect to vary the phase difference between the reference and comparative oscillations from the aforesaid predetermined relation, thereby to phase-modulate the comparative oscillations in accordance with themodulation signals.' l

In accordance with one form of the invention, a phase-modulation system of the type described comprises means for deriving from one of the reference or comparative oscillation-generating means oscillations of saw-tooth wave form, means for deriving from the other generating means a potential of periodic-pulse wave form having a period and phase corresponding to the oscillations of the said other generating means, and means responsive to the relative phase of the derived potential and the oscillations of saw-tooth Wave form for deriving the aforesaid control effect.

In accordance with a particular form of the invention, the phase-modulation system includes a carrier Wave source and means responsive to the carrier wave of the source for deriving comparative oscillations of a sub-multiple frequency thereof. Preferably, the aforesaid oscillations of saw-tooth Wave form are'derived from the reference oscillations and the 'aforesaid potential of periodic-pulse wave form is derived from the comparative oscillations. i

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended. claims.

Referring noa1 to the drawings, Fig. 1 is a circuit diagram, partly schematic, cfa complete frequency-modulation transmitter embodying a phase-modulation system in accordance with the present invention; Fig. 2 is a graph used in explaining the operation of the Fig'. 1 arrangement; Fig. 3 is a circuit diagram, partly schematic, of a complete frequency-modulation carrier-wave transmitter embodying a modified form of the invention; and Fig. 4 is a graph used as an aid in explainingthe operation of the Fig. 3 transmitter arrangement.

Referring now more particularly to Fig. 1, there is represented schematically a complete frequency-modulation carrier-Wave transmitter of a conventional design embodying the present invention in a particular form. In general, the

transmitter includes a first source of modula` tion signals, comprising a microphone IIi, an audio-frequency amplifier II of one or more stages, an integrating system I2, and an audiofrequency amplifier I3 of one or more stages. The modulation-signal source comprising units I to I3, inclusive, is coupled to the inputV circuit I8, I9 of a phase-modulation system Iii, more fully described hereinafter. [There is coupled to the output circuit of unit I?! one or more frequency-multiplier and radio-frequency amplier stages I5, and an antenna system I6, II. The transmitter may also include a second source of modulation signals, comprising a microphone Iii', an audio-frequency amplifier II' of one or more stages, an integrating system I2', and an audio-frequency amplier I3 of one or more stages which is coupled to asecond input circuit comprising input-circuit terminals I8', I9' Of the phase-modulation system I4.

. derived by the microphone I0, is amplified by the audi-frequency amplifier I I, is suitably integrated by the integrating system I2, and after further amplification in the audio-frequency amplier I3, is applied to an input circuit of the phasemodulation system I4. The phase-modulation system IIA phase-modulates a carrier Wave generated therein in accordance with the modulation signals applied to its input-circuit terminals I8, I9 from the modulation-signal source I0, II, I2, I3 and the phase-modulated carrier wave is applied to the input circuit of unit I5, where its frequency is increased by the frequency-multiplier stages of this unit to a value suitable for radiation by the transmitter. quency carrier wave is then amplied by the radio-frequency amplifier stage or stages of unit I5 and is applied to the antenna system I6, II for radiation.

The modulation signal derived by units II', I2' and I3 and applied to the second input circuit comprising input-circuit terminals I8', I9' of thephase-modulation system Idmay simultaneously or alternately phase-modulate the carrier wave generated in unit I 4, the operation of the transmitter after the phase modulation of the carrier wave being as heretofore described.

Referring now more particularly to the portion of the system embodying the present invention, the phase-modulation system Id includes an input circuit comprising input-circuit terminals Iii, I9 adapted to have applied thereto modulation signals to be transmitted. The system also includes means for generating reference oscillations comprising a piezoelectric crystal-controlled oscillator 2li having a frequency, for example, ci kilocycles. There is also included in the system means for generating comparative oscillations of the mean frequency of the reference oscillations comprising an oscillator 2I, the frequency of which is determined by an output tuned circuit comprising a condenser 22 vand inductor 23.

There is additionally included in the phasemodulation system Id means responsive to the relative phase of the reference and comparative oscillations, generated by the respective oscillators 20 and 2l, for deriving a control effect. This means comprises means for deriving from one of the generating means 2li, 2l oscillations of saw-tooth wave form, means for deriving from the other of the generating means 2li, 2| a potential of periodic-pulse wave form having a period and phase corresponding to the oscillations thereof, and means responsive tc the l relative phase of the derived potential and the saw-tooth oscillations for deriving the control` effect. In the particular arrangement shown, the means for generating the oscillations'of saw-tooth wave form comprises a saw-tooth wave generator 26 having an input circuit coupled to the outputA The 4high-fref uum tube 2l and the resistor 26 being connected between the control' electrode 21 andthe cathode of this vacuum tube. The means responsive to the vrelative phase of the reference and comparative oscillations for deriving the control eect comprises the vacuum tube 28, which includes a i second control electrode 29 coupled through a condenser 30 to the output circuitof the sawtooth generator 24, and a rectifier device 2| which is coupled through a transformer 32 to the out` putcircuit of the vacuum tube 28. The transformer 32 has' a primary winding 33 which is tuned by a condenser 34 to the frequency of the oscillator is of the piezo-electric crystal-conf,

reference oscillations generated by the oscillator 20. the tuned circuit 33, having a pass band sufficiently wide to translate to the rectier 2| the fundamental frequency and perhaps several harmonic frequencies of pulses derived, Ain a manner presently to be explained, from the saw-tooth oscillations applied to tube 28 from the generator 24. '111e rectifier 3|' is provided with a load resistor 25 across which there is derived the aforementioned control e'ect which, as here shown,l is a unidirectional control potential.` There 'is connected across the resistor 35 a condenser 36 of such valuethat the resistor-35 and` condenser It have a time constant long compared with the period ot the lowest frequency of the oscillator 2|. 'I'he phase-modulation system il additionally includes means responsive to the aforesaid con-,1j

trol eiect. tending to maintain'apredetermined constant phase relationbetween the reference and comparative oscillations.- This means com' anode current of vacuum tube 28 is consequently lation system I4 and included in a series-biasing circuit for the control electrode 38 `of the reactance tube 31, the latter circuit comprising a source of'bias potential `Ci, the resistor the Vload resistor 35 of rectifier device 3|, and the l resistor 39.

Considering now the operation of the circuit -just described, and referring to the curves of Fig.

2,' the reference oscillations generated by the oscillator 20 have a constant frequency-since this trolled type. The saw-,tooth generator 24 derives directly .from the reference oscillations saw-tooth oscillations and it will be apparent that the latter are likewise of constant frequency, have-a constant phaseV relation with respect to the'reference oscillations, and preferably are of linear saw-tooth wave forni,` as representedby` curve A of Fig. 2. AThese 'saw-tooth oscillations are applied through the coupling condenser 3|). 'to the control electrode of the vacuum tube 28.

The control electrode 221sbiased to the linear portion oi its operating characteristic by the source of biasing potential -C. The tube 28, however, is normally biased to cutoff during the major portion of the period of the saw-tooth oscillations by a negative-bias potential developed acrossthe resistor 26 by the action of thecon-` l trol electrode 21 in rectifying the comparative.

oscillations, represented by the solid-line curve B of Fig. 2 applied .thereto through the condenser 25 from the oscillator 2|-.- 'I'he control electrode 21 is biased above cutoff cfg-Fig. 2, only by the positive peaks of the comparative oscillations and o thus permits anode current to flow during a relatively. short intervallli-ts. ti-tt, etc., of each cycle of the comparative oscillations.

The sawtooth oscillations applied tothe control electrode 29' are thus repeated in the 'output circuit of .vacuum tube 28 only during these intervals. The

of periodic-pulse wave form, as represented by the solid-line curve C offFlg. 2. The output arranged. the reactance tube $1 comprises means 'l responsive to the control effect derived across the load resistor Il ofthe rectiner device 3| for controlling the'frequency ofthe oscillator 2l and tending to maintain a predetermined constant phase relation between the reference4 and coniparative oscillations generated by the respective oscillators v2l and 2|.

The phase-modulation system also includes means responsive to the modulation signals for modifying the action of the aforementioned con-.- trol teilect to'vary the vphase difierencebetween the reference and' comparativejoscillations from the aforesaid predeterminedrelation. thereby to phase-modulste-the comparative oscillations of oscillator 2| in accordance with the modulation .Signals applied to unit Il from the modulationsignal'source Il, I'i, |2 and It. 'Insofaras the modulation signals derived bythis source are concerned, this means'comprises a resistor Il which is coupled across the input-circuittennii nels ls. is ofthe phase-modulation system u and is included in a series-biasing circuit for lthe con' trol electrode 2l of the vacuum tube 2l. the lat-- ter circuit comprising. in addition to the resistor tion signals derived by the modulation-signal source Il'. I2 and i3' are concerned. the

source of biasing potential C and a radiofrequency choke coil u. Insofar as the modulamodulation-signal responsive means comprises a.

resistor Il coupled across the second pair of input-circuit terminals Il', Il' of the phase-modugenerator 26. n It will be evident that, should the phase o! pulses of vacuum tube 28 are peaky rectified by the rectiner device Si to derive across the load 'resistor It a unidirectional control potential.

This control potential is applied in series with the biasing potential from the biasing source -Ci tothe control electrode 38 of the reactancetube 31 and the biasing potential -Ci is so adjusted that'the effective reactance applied across the tuned circuit 22,-28 of the oscillator 2|' from the reactance tube t1 is just suilicient to maintain theoutput pulses of tube :28 at that constant amplitude which corresponds to a-'predetermined constant. phase relation between the reference oscillations ofoscillator 2t and the comparative oscillations of oscillator 2|. 'I'liis constant phase relation is preferably suii that the positive peaks "of the oscillations of oscillator 2| occur at the center 'point a of the linear portion of the sawtooth oscillations generated by the the oscillations of oscillator 2| advance with revspeci; to the reference oscillations of oscillator 2li, as indicated 'by the dotted-line curve B, Hg. 2,

the periodic-pulse current in the output circuit 'of vacuum tube 28 has lower amplitude. as represented by the dottedpline curve C'. Hence. the

unidirectional potential derived across resistor 25Y is smaller than before and .the etlective reactance of the reactance'tube 21 in a direction -4 to retardthe phase of the 'comparative oscillations of oscillator y2|-` in an attempt to restore the saw-tooth oscillations."

l its initial value.

relative Jphase of the reference and comparative tiveand'r'eference oscillations have substantially the predetermined phase initially established.

Similarly,.should the comparative oscillations of oscillatorl 2| be delayed` in phase with respect to the "referenceoscillations, ofioscillator 20, as indicatedby the broken-line curve B" of Fig. 2,- the periodic-pulse` current in they output circuit of vacuum tube 23 has larger amplitude, as represented by the broken-line curve C. with resultant increase of: the unidirectional potential developed across the Vresistor-35, thereby to change the effective reactance of the reactance tube 31 to increase the frequency'of `'the comparative os` cillations generated by oscillator 2 I thus substantially to restore'the initial predetermined phase relation between the reference and comparative Assume now `that amodulation signal is derived by the modulation-signal source comprising units ||l, I, I 2 and I3 and is applied to the input-circuit terminals I8, i9 of the phase-modulation system |4. vA modulation signal potential is thus 'developed across the resistor 43 and is applied to the control electrode 29 of the vacuum tube 28. This has the same effect on the instantaneous potential of the control electrode 29 as though the phase of the saw-,tooth oscillations of oscillator 24 were instantaneously advanced or retarded, in accordance with the polarity of the modulation signal, with respect t'o the comparative oscillations of oscillator-2|. 'I'he relative phase of the saw-tooth oscillations is advanced or retarded, respectively, byipositive and negative values of the modulation signal and the extent of the change in phase varies with the instantaneous amplitude of the modulation signal. The resultant change of the unidirectional control potential developed across the resistor 35 effects a corresponding change in the effective reactance of the reactance tube 31 to cause. the comparative oscillations of the oscillator. 2| correspondingly to change in phase in such direction that the magnitude of the control potential developed across the resistor 35 is changed toward It will be evident, of course. that the magnitude of the unidirectional control potential is not restored to its initial value by the phase change of the comparative oscillations, but rather is changed to such` value that an equilibrium value of phase is established between the reference and comparative oscillations.

However, since the actual phase of the saw-tooth oscillations of generator 24 has not changed, it will be apparent that the effect of the modulation signal is so to modiiyjthe action of the phase-controlling effect or potential developed across the resistor 35 as to vary the phase difference between the reference and comparative oscillations from the predetermined phase relation initially established therebetween, thereby to phase-modulate the comparative oscillations of oscillator 2| in accordance with the modulation signals.

It will be apparent from the above description of the operation of the invention that modulation signals derived by the modulation-signal source comprising units I0', l2' and I3' and applied to the second input-circuit terminals y I8', I9 o f the phase-modulation system I4 likewise effect a variation of the phase diiferenceibetween the reference and comparative oscillations from the predetermined phase relation' initially established.thereby to phase-modulate the comparative oscillationsV` ,in accordance 'with the modul'ttionsig'nalsv derived. by the microphone v| Il. The operation inthis respect is essentially similar 'to'thatpreviously described except that the latterl modulation signals are directly applied in serieslwiththe control potential developed across .the'load'resistor 35 to the control electrode 38 of the reactance tube 3'I, thus to modify the eiect Y of the control potential in maintaining the initial predetermined phase relation between the reference and comparative oscillations. A

- While thecomparative oscillations of oscillator 2| may thus be simultaneously phase-modulated in accordance with modulation signals applied to both of the pairs of input-circuit terminals I8, I9 and I8', I9' of unit I4, normally only one 4of vthe modulation-signal sources comprising units ||l to I3, inclusive, or I0' to |3, inclusive, will b e used at any one time,`the other source being temporarily deenergized by suitable means or being completely eliminated from the transmitter arrangement. v

It will now be evident from the foregoing described operation of the invention that the linearity of wave form of the saw-tooth oscillations generated by unit 24 directly determines the linearity of they phase modulation of the comparative oscillations o`f oscillator 2|. It is relatively easy in accordance with present day practice t obtain saw-tooth oscillations having a high degree of linearity of wave form. Consequently, a transmitter embodying the invention has the advantage that a high degree of linearity of phase modulation of the comparative oscillations may easily be attained. It will further be evident that the phase modulation of the comparative oscillations may extend over a relatively wide range equal to a phase deviation of :e170

degrees withrespect to the reference oscillations of oscillator 20 if 20 degrees of the saw-tooth oscillation are allowed for the retrace portion thereof and a reasonable value is allowed for the duration of the pulses of the periodic potential derived from the comparative oscillations of oscillator 2| This has the advantage that fewer frequency-multiplier stages are required in unit I5 of the transmitter to obtain the high-frequency frequency-modulated carrier wave desired for radiation. For example. it can be shown`that:

where msx=the maximum `phase deviation corresponding to the maximum frequency deviation desired,

Afmx=the maximum frequency deviation sired, andA fa=the lowest-frequency component of the modulation signal. 1

lThus, if the comparative oscillations are phaseorder o; 5000 times has been required in the prior .art phase-modulation systems for the same maximum frequency deviation and the same value of low-frequency component of the modulation signal, since vphase modulation ofthe low-frequency oscillations in such systems could only bev effected over approximately plus or'minus onehalf radian thereof if any semblance of linearity of phase modulation was to be retained.

`Fig. 3 represents a modified form of phasemodulation system of the invention which is essentially similar to that of Fig. 1, similar circuit elements being designated by similar reference numerals and analogouscircuit elements by similar reference numerals primed, except that a modified form of ,phase-modulation control is employed in the instant arrangement and fewer frequency-multiplication stages are required to produce the high-frequency frequency-modulated wave to be radiated. The phase-modulation control system in this arrangement comprises a pair of control vacuum tubes 20, 28' having a pair of rectifier devices 3i, 3|' individually coupled to the output circuits thereof. Each of the rectifier devices 3i, 3|' has an output load impedance comprising respective load resistors 35, 35' and respective parallel-connected condensers 36, 30,.the load resistors 35 and 35' being serially included in the biasing circuit of the control electrode 38 of the reactance tube 31. The saw-tooth Wave 'generator 2d' developsr balanced saw-tooth oscillations in its output circuit, which oscillations are applied with one phase through the` condenser it to the control e1ectrode'29 of vacuum tube 2t' and with opposite phase through a condenser 30 to the control electrode 29 of vacuum tube 28. The modulation signals applied to the input-circuit terminals i8, i9 of the phasemodulation system it are applied through a modulation transformer 46 with opposite phase to the control electrodes '29,and 29 of the respective vacuum tubes 28 and 28p'.

Also in the arrangement of Fig. 3, the oscillator 2i' comprises a carrier-wave source, the resonant circuit 22', 23' thereof being tuned to a frequency higher than that of the reference oscillator 20. There is included means responsive to the Vcarrier wavegenerated by the oscillator 2i' for deriving comparative oscillations of a submultiple frequency thereof, the mean .frequency of the comparative oscillations being equal to the frequency of the reference oscillations ofioscillator` 20. This means is essentially a frequency divider and comprises a vacuum tube t1 having a control electrode 48 connected to ground through a grid-leak resistor 69 and coupled through, a condenser 50 to the output circuit of the carrier-wave oscillator 2i'. The output circuit of the vacuum tube 41 comprises a ytransformer winding 53 which is tuned by a condenser Et toone-fourth of the mean frequency of the carrier wave applied to the input circuit of vacuum tube t1. A transformer winding 55 is coupledto the Winding '53` and is connected to a control electrode 56 of a vacuum tube 51. The output circuit ofthe vacuum tube51 comprises a primary winding 58-of a transformer 59, the secondary winding 50 of which is tuned by a condenser ti to three-fourths of i the mean frequency of the carrier wave generated by oscillator 2l'. 'Ihe transformer secondary winding 60 is coupled to a. second control electrode 62 of vacuum tube e1. A tertiary transformer winding 63 is coupled to the winding 53 included in the output circuit of vacuum tube 41 and is coupled through the condenser 25 to the control electrode 21 of vacuum tube 28 and to a corresponding control electrode 21 of vacuum tube 28'.

`Considering now the operation of the circuit just described, and referring to the curves of Fig. 4, `the saw-tooth oscillations generated byv unit 24 and applied to the control electrode 29 vof vacuum tube 28 are represented by the solid- -line curve D, while the saw-tooth oscillations applied to the control electrode 29 of vacuum tube 28 are represented by the broken-line curve E. The carrier wave generated by the carrierwave oscillator 2l' has, in the arrangement described, a, mean frequency four times that of the reference oscillations generated by oscillator 20. There is derived fromy this carrier wave by the frequency-divider stage comprising vacuum` tubes 41 and 6l comparative oscillations of a submultiple frequency of the carrier wave and having a mean frequency equal to that of the reference oscillations. These comparative oscillations are applied through the transformer winding 63 and through the condenser 25 in like phase to the control electrodes 21 and 21' of the respective vacuum'tubes 23 and 28. As in Fig. 1, the control electrodes 21 and 21 rectifythe comparative oscillations to derive across the resistor 25 a bias potential suiiiciently large that the vacuum tubes 28, 28 are conductive only during a rela-tively short interval of each positive peak of the comparative oscillations when the control electrodes are instantaneously biased above their cut-oli" bias' eo. as indicated by the portion of the solid-line curve F of Fig. 4. Consequently, the saw-tooth oscillations applied to the control electrodes 29 and 29 of the respective vacuum tubes 28 and 28' are repeated in.

by theV rectier device 3| to develop across thev 10aa rsister as a first umdirecuonn potenun.

1 Similarly, the current in the outpu-tv circuit of tube 28'l is of periodic-'pulse Wave form, as represented by the solid-line curve G below the axis O O, and there is derived across the load resistor 35' a second unidirectional control poten- 4tial of. polarity` opposite to that of theiirst uni- `directional control potential. When the reference and comparative oscillations applied-to the vacuum tubes 28 and 28' havethe phase rela tion indicated in Fig. 4, the unidirectional control potentials derived'across the resistors 35.

and 35 are equal and thus exactly oppose each other in the biasing circuit of the Acontrol electrode 38 of the reactance -tube 31. I'he bias C1 is initially so adjustedI as in the arrangement of Fig. 1 that the reference and comparative oscillations have a desired predetermined phase relationship under the conditions just assumed.

Now should the frequency of the carrier'wave generated by the oscillator 2| change, the phase of the comparative oscillations'produced by the frequency divider comprising tubes 41 and 51 correspondingly changes, though to a lesser degree 4due to the frequency division, with respect to the reference oscillations of oscillator 20.

lations, as represented by the broken-line curve` F' of Fig. 4, the 'output of the vacuum tube 28 is reduced, as indicated by the portion of the dotted-line curve G' extending above the axis 0 0,

while the output of the vacuum tube 28' increases, as represented by the dotted-line curve G' belowthe axis o-IL Consequently, the unidirectional control potential developed across the resistor 35' is larger than that developed across the resistor 35 with the result that a control bias is applied to the control electrode 38 of the reactance tube 3l to cause a change of the eiective reactance of the latter in a direction to change the frequency of the'carrier-wave oscillator 2l' in an attempt to restore equality of the unidirectional potentials derived across the resistors 35 and 35'. As previously pointed out, however, sufficient inequality of the unidirectional potentials is required to eiect a state of equilibrium between the phasey of the reference and comparative oscillations. In this connection, it may bev noted that the reactance tube 81 must cause a phase. shift of the carrier-wave oscillations of oscillator 2 l four times as large as that required of. the comparative oscillations to effect the desired balance of outputs of the rectifier devices 3l and 3l. The arrangement of Fig. 3 thus produces a large phase shift of the carrier-wave oscillations generated by oscillator 2| for relatively small phase diierences between the reference and comparative oscillations.

It will be evident that, should the phase of the comparative oscillations be retarded with respect to the reference oscillations, as indicated by the broken-line curve F" of Fig. 4, the output of the vacuum tube 28 is now increased and that of vacuum tube 28 decreased, as represented by the portions of the broken-line curve H respectively above and below the axis 8 0. The resultant of the control potentials now developed across the resistors 35 and 35' has a polarity opposite to that previously described and which resulted from yan, advance in phase of the comparative oscillations with respect to the reference oscillations. The phase correction applied to the carrier-wave oscillator 2l' by the reactance,

tube al is thus opposite to that` previously described and again eiects substantial' equality of outputs of the -rectiiders 3l and 35i'.

The eect of the modulation signals applied to the input circuit terminals I8, I9 of unit Ui' in the Fig. 3 arrangement is similar to that described in connection with Fig. 1 in that it has the same eect asv though the saw-tooth oscillations applied to the control electrodes 28 and 29' of the respective vacuum tubes 28 and 28' were changed in phase, though vthere is the difference that the modulation signals have the effect of advancingthephase of the saw-tooth oscillations applied to one of the tubes, for example the tube 28, and of retarding the phase of the-saw-toothoscillations applied to the other, for example the tube 28', thus producing an unbalance of the outputs of the rectifier devices 3| and 3l' of the same type as though the comparative oscillations had changed in phase with 'respect to the reference oscillations. The operation Aof the Fig. 3 arrangement is otherwise essentially similar to that of Fig. 1 and will not be repeated,

The arrangement of Fig. '3 has the advantage that the phase of the carrier-wave oscillations of oscillator 2l' is varied over a wider yphase range by a modulation signal ofgiven amplitude than is the case in the arrangement of Fig. 1. The Fig. 3 modiiication thus requires fewer stages of frequency multiplication in unit I5 of the transmitter to produce the required high-fre-` quency frequency-modulated carrier wave for radiation.

It will be apparent from the foregoing description that themean fundamental frequen` cies of the reference and comparative operations are integrally'related. In the Fig. l embodiment, for example, these oscillations have the same mean fundamental frequency. It will also be apparent that a phase-modulation system in accordance with the invention comprises means responsive to the timing of corresponding cycles of the reference and comparative oscillations 'which have the same mean time separation for deriving a control effect. As used here, the term corresponding" is intended to designate a correspondence in time. Referring again to the Fig. 1 ernbodiment, this means comprises vacuum tube 28, diode 8i, and their associated circuits.4 Where, as in the embodiment under consideration, the reference and comparative oscillations have the same mean fundamental frequency, succeeding cycles of the reference oscillations have the same mean'time separation as succeeding cycles of the comparative oscillations so that'the means comprisedof vacuum tube 28 and diode 3l are responsive to corresponding and successive cycles oi the reference and comparative oscillations, as illustrated by the curves of Fig. 2. Expressed in other words, the described means are responsive to the relative phase of the reference and comparative oscillations for the Istated operating condition and the derived control eiect is utilized to maintain a predetermined constant timing or phase `relation between corresponding `cycles of the reference and comparative oscillations.

While there have been described what are at presentaconsidered tobe the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and Y modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover al1 such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A phase-modulation system comprising, an

input circuit adapted to have applied' theretoL modulation signals, means for generating reference oscillations, means for'generating compara-- tive oscillations of the mean frequency of said reference oscillations, means responsive to the relative phase of corresponding cycles of the reference and comparative oscillations for deriving a control effect, means responsive to said control eilect tending to maintain a predetermined constant phase relation between said reference and comparative oscillations, and means responsive to said modulation signals for modifying the action of said control effect to vary the phase difference between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance with said modulation signals.

2. A phase-modulation system comprising, an input circuit adapted to have applied thereto modulation signals, means for generating reference oscillations of substantially constant frequency. means for generating comparative oscillations oi the mean frequency of said reference oscillations, means responsive to the relative phase oi correspondingl cycles `of the reference and comparative oscillations for deriving a control eifect, means responsive to said control eiIect tending to maintain a predetermined constant phase relation between said reference and comparative oscillations, and means responsive to said modulation signals for modifying the action of said citrol eect to vary the phase dierence between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance with said modulation signals.

3; A phase-modulation system comprising, an input circuit adapted to have applied thereto modulation signals, means for generating reference oscillations, means for generating comparative oscillations of the mean frequency of said reference oscillations, means for deriving from one of said generating means oscillations of sawtooth wave form, means responsive to the relative phase of the oscillations of the other of said generating means and said saw-tooth oscillations for deriving a control eiect, means responsive to said control effect tending-to maintain a predetermined constant phase relation between said reference and comparative oscillations, and means responsive to said modulation signals for modifying the action of said control eilect to vary the phase-difference between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulatesaid comparative oscillations in accordance with said modulation signals.

4. A phase-modulation system comprising, an input circuit adapted 'to` have applied thereto modulation signals, means for generating reference oscillations, means for generating comparative oscillations of the mean frequency of said reference oscillations, means for deriving from one of said generating means oscillations of linear saw-tooth wave form, means responsive to the relative phase of the oscillations of the other of said generating means and said saw-tooth oscilence oscillations, means for generating comparative oscillations of the mean'frequency of saidv reference oscillations, means responsive to the lations for deriving a control effect varying linearly therewith, means responsive to said control eiect tending to maintain a predetermined constant phase relation between said reference and comparative oscillations, and means responsive to said modulation signals for modifying the action of said control eiect to vary the phase difference between said reference and comparative oscillations from said predetermined relation,

thereby linearly to phase-modulate said comparative oscillations in accordance `with lation signals.

l 5. A phase-modulation system comprising, an inputrcircuit adapted to have' applied thereto modulation signals, means for generating reference oscillations, means for generatingcomparative oscillations of the mean frequency of said reference oscillations, means responsive to the said modurelative phase of corresponding cycles oi' the reference and comparative oscillations for deriving a unidirectional control potential, means responsive to said control potential tending to maintain a predetermined constant phase relation between said referenceand comparative oscillations, and means responsive to said modulation `signals for modifying the action of said unidirectional control potential to vary the phase diilerence between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative osrelative phase of corresponding cycles of the reference and comparative oscillations for `deriving a unidirectional control potential having an amplitude varying with said relative phase, means responsive to said control potential tending to maintain apredetermined constant phase relation between said reference and comparative oscillations, and means responsive to said modulation signals for modifying the action of said control potential to vary the phase difference vlietween said reference and comparative oscillations from* said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance with said modulation signals.

7. A phase-modulation system comprising, an input circuit adapted to have applied thereto modulation signals. means for generating reference oscillations, means for generating comparative oscillations of .the mean frequency of said reference oscillations, means for deriving from the osclllationsof one of said generating means Va potential of periodic-pulse wave form having a period and phase corresponding to the 'oscillations of said one generating means, means responsive to the relative phase of saidderived potential and the oscillations of the other of said generating means' for deriving a control effect, means responsive to said control effect tending to maintain a predetermined constant phase relation between said reference and comparative4 oscillations, and means responsive to said4 modulation signals for modifying the action of said control effect to vary the phase difference between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillationsin accordance with said modulationsignals.

8. A phase-modulation system comprising, an

input circuit adapted to have applied theretoA a potential of periodic-pulse wave form having substantially constant amplitude andfhaving a period and phase corresponding to the oscillations of said one generating means, means for generating from the oscillations of the other of said generating means oscillations of saw-tooth wave form, means responsive to the relative phase of said derived potential and said saw-tooth oscilv lations for deriving a control effect, means responsive to said control eilect tending to maintain a predetermined constant phase relation between said reference and comparative oscillations, `and means responsive to said modulation signals for modifying the action of said control effect to vary the phase difference between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance wlthsaid modulation signals.'

9.A A phase-modulation system comprising, an

input circuit adapted to have applied thereto modulation signals, means for generating refer- 4 ence oscillations, means for generating comparacillatio'ns in accordance with said modulationV tive oscillations of the mean frequency of said reference oscillations, means responsive to said comparative oscillations for deriving a potential of periodic-pulse wave form having a period and phase corresponding to said comparative oscillations, means responsive to said reference oscillations'for deriving oscillations of saw-tooth wave form having the same period as said reference oscillations, means responsive to the relative phase of said derived potential and said saw-tooth oscillations for deriving a control eiiect, means responsive to said control effect tending to maintain a predetermined constant phase relation between said reference land comparative oscilla-l tions, and means responsive to said modulation signals for modifying the action of said control effect to vary the phase difference between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance with said modulation signals.

10. A phase-modulation system comprising, an input circuit adapted. to have applied ,thereto modulation signals, means for 'generating reference oscillations, means for generating comparative oscillations of the mean frequency oi said reference oscillations, vacuum-tube means responsive to the relative phase of corresponding l cycles of the reference and comparative oscillative oscillations of the mean frequency of saidI reference oscillations, means including a. vacuum tube having two control electrodes individually coupled to said first-namedl and second-named generating means for deriving a control eiect varying in magnitude with the relative phase of corresponding cycles of said reference and comparative oscillations, vmeans responsive to said control effect tending to maintain a predetermined constant phase relation between said reierence and comparative oscillations, and means responsive to said modulation signals for modifying the action of said control effect to vary the phase difference between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance with said modulation signalsl 12. A phase-modulation system comprising, an input circuit adapted to have applied thereto modulation signals, means for generating reference oscillations, means for generating comparative oscillations'of the mean frequency of said reference oscillations, means for deriving from the oscillations of one of said generating means oscillations of saw-tooth wave form having the same period as the oscillations of said one generating means, means ior deriving from the oscillations of the other of said generating means a potential of periodic-pulse wave form having the same period as the oscillations of said other generating means, means including aI vacuum tube having a pair of control electrodes individually coupled to said first-named and second-named deriving means for deriving a control effect vary- Qing in magnitude with the relative phase of said derived potential and said saw-tooth oscillations, means responsive to said control effect tending to maintain apredetermined constant phase relation between said reference and comparative oscillations, and means responsive to said modulation signals for modifying the action of said control eiect to vary the phase difference between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance with said modulation signals` 13. A phase-modulation system comprising, an

-input circuit adapted to have applied thereto modulation signals, means for generating reference oscillations, means for generating comparative oscillations of the mean frequency of said reference oscillations, means for translating the oscillations of one of said generating means only during a fixed and relatively short interval of each cycle of the oscillations of the other oi said generating means, means responsive to said translated oscillations for deriving aA control eiect, means responsive to said control etect tending to maintain a predetermined constant -phase relation between said reference and comparative oscillations, and `means responsive to said modulation signals for modifying the action of said' control effect to vary, the phase dierence between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance with said modulation signals.

14. `A phase-modulation system comprising, an input circuit adapted4 to have applied thereto modulation signals, means ior generating reference oscillations, means including an oscillator for generating comparative oscillations of the mean frequency of said reference oscillations, means responsive to the relative phase of corresponding cycles of the reference and comparative oscillations for deriving a control eiect, means responsive to said control efiect for controlling the frequency of said oscillator to tend to maintain a predetermined constant phase relation between said reference and comparative oscillations, and means responsive to said modulation signals for modifying thevaction of said control effect in controlling the frequency of said oscillator to vvary the phase difference between said reference and comparative oscillations from said predetermined relation, thereby to phase-modulate said comparative oscillations in accordance with said mod ulation signals.

15. A phase-modulation system comprising, an input circuit adapted toihave applied thereto modulation signals, a carrier-wave source, means responsive to the carrier wave of said source for deriving comparative oscillations of a submultiple frequency thereof, a source of reference oscillations having a frequency equal to the mean frequency of said comparative oscillations, means responsive tothe relative phase of corresponding cycles of said reference and comparative oscillations for deriving a control enact, means responsive to said control eect for controlling the irequency of said carrier wave to tend to maintain a predetermined constant phase relation leetween said reference .and comparative oscillations, and means responsive to said modulation signals for modifying the action oi said control eiiect in controlling the frequency of said carrier Wave to vary the phase difference between said reference and comparative. oscillations from said predetermined relation, thereby to phase-modulate said carrier wave in accordance with said modulation signals.

16. Aphase-modulationl system comprising, an input circuit adapted to have applied thereto modulation signals, a carrier-wave source, -means responsive to the carrier wave of said source for deriving comparative oscillations of a submultiple frequency thereof, a source 'of reference oscillations of saw-tooth Wave form having a frel quency equal .to the mean frequency of said comtrolling the frequency of said carrier wave. to

vary the phase difference between said reference and comparative Aoscillations from said predeter-` mined relation, thereby to phase-modulate said,

carrier wave in accordance with said modulation signals. l q l 17. A phase-modulation system comprising, an

input circuit adapted to have applied thereto modulation signals, means for generating reference oscillations, means for generating comparative oscillations, ,the mean fundamental frequencies of said reference and comparative oscillations being integrally related, means responsive to the timing of corresponding cycles of said reference and comparative oscillations which have the same mean time separation for deriving a control.

thereby to phase-modulate said comparative osl l' effect, means responsive to said control effect` tending to maintain a predetermined constant timing between said corresponding cycles of said reference and comparative oscillations, and

means responsive to said modulation signals'for modifying the action 0f said control effect to vary the timing between said reference and comparative oscillations from said predetermined timing,

cillations in accordance with said modulation signals.

18. A phase-modulation system comprising, an

input circuit adapted to have applied thereto modulation'signals, means for generating reference ,oscillationsof substantially constant frequency, means for generating comparative oscillations having a mean fundamental frequency integrally related to the frequency of said reference oscillations, means responsive to the timing of corresponding cycles of said reference and comparative oscillations which have the saniemean time separation for deriving a. control effect,

means responsive to said control eiect tending to maintain a'predetermined constant timing between said corresponding cycles of said reference and comparative oscillations, and means responsive to said modulation signals for modifyingthe action of said control eifect .to vary the timing between said reference and comparative oscillations from said predetermined timing, thereby to phase-modulate said comparative oscillations in accordance with said modulation signals.

19. A phase-modulation system comprising,ian input circuit Vadapted to have applied thereto modulation signals, means for generating reierence oscillations, means for generating comparative oscillations, the mean fundamental frequen- -cies of said reference and comparative oscillations being integrally related, meansior deriving from the oscillations of one vof said generating means pulses of potential having a period and phase corresponding to the oscillations of vsaid 'one generating means, means responsive to the timing of said pulses and corresponding cycles of `the oscillations of the other of said generating means which have the same mean time separation for deriving a control effect, means responsive to said control eifect tending to maintain a predetermined constant timing between corresponding cycles of said reference and comparative oscillations, and means responsive to said modulation signals for modifying the action of said control eifect to vary the vtimingbetween said reference and comparative oscillations from said predetermined timing, thereby to phasemodulate said comparative oscillations in accordance-with said modulation signals.

BERNARD'D. LOUGHLIN. 

